1. Field of the Invention
This invention relates to a self-pulsation semiconductor laser.
2. Description of the Related Art
For using a semiconductor laser as a light source of an optical disc device, or the like, it is important to find how to suppress noise caused by return light. A known approach for suppressing return light noise is to use a so-called self-pulsation semiconductor laser configured to oscillate by self-excitation to cope with multi-mode operation. FIG. 1A is a cross-sectional view of such a conventional self-pulsation semiconductor laser. The self-pulsation semiconductor laser operative is shown here as being made of AlGaInP-based materials.
As shown in FIG. 1A, the conventional AlGaInP-based self-pulsation semiconductor laser is made by sequentially stacking on an n-type GaAs substrate 101 an n-type AlGaInP cladding layer 102, GaInP active layer 103, p-type AlGaInP cladding layer 104, p-type GaInP intermediate layer 105, and p-type GaAs cap layer 106. An upper-lying portion of the p-type AlGaInP cladding layer 104, p-type GaInP intermediate layer 105 and p-type GaAs cap layer 106 are made in a mesa-type stripe configuration extending in one direction. Numeral 107 denotes the stripe portion including the upper-portion of the p-type AlGaInP cladding layer 104, p-type GaInP intermediate layer 105 and p-type GaAs cap layer 106. Buried at both sides of the stripe portion 107 is an n-type GaAs current blocking layer 108 to thereby form a current blocking structure.
Formed on the p-type GaAs cap layer 106 and the n-type GaAs current blocking layer 108 is a p-side electrode 109 made of, for example, Ti/Pt/Au. On the other hand, an n-side electrode 110 made of AuGe/Ni/Au, for example, is formed on the bottom surface of the n-type GaAs substrate 101.
FIG. 1B is a schematic diagram showing distribution of refractive indices of the conventional AlGaInP-based self-pulsation semiconductor laser. In this diagram, distribution of refractive indices of the AlGaInP-based self-pulsation semiconductor laser, in a particular direction parallel to the p-n junction of and normal to the cavity lengthwise direction (the particular direction is hereinafter referred to as the transverse direction), is shown in a correspondence with FIG. 1A.
That is, as shown in FIG. 1B, the conventional AlGaInP-based self-pulsation semiconductor laser exhibits step-shaped distribution of refractive indices in the transverse direction in which the refractive index n.sub.1 ' at the portion corresponding to the stripe portion 107 and the refractive index n.sub.2.sup.1 ' at the portion corresponding to both sides of the stripe portion 107. In the conventional self-pulsation semiconductor laser, the transverse optical guide is effected by stepwise changing its transverse refractive index. In this case, the refractive index difference .DELTA.n'(=n.sub.1 '-n.sub.2.sub.2 ') between the the portion corresponding to the stripe portion 107 and the portion corresponding to both sides of the stripe portion 107 is chosen to be approximately 0.0003 or less, which is nearer to the value of a gain-guided semiconductor laser, 0, than the typical value of an ordinary real-index-guided semiconductor laser, 0.001. In this manner, the transverse optical confinement of the GaInP active layer 103 is made moderate.
During operation of the conventional self-pulsation semiconductor laser of this type, the width W.sub.P ' of the optical guide region 112 relative to the width W.sub.G ' of the gain region 111 in the GaInP active layer 103 becomes large, and a part of the optical guide region 112 outside the gain region 111 becomes a saturable absorbing region 113. In the conventional self-pulsation semiconductor laser, self-pulsated oscillation is realized by reducing the change in transverse refractive index to increase the extruded amount of transverse light and to enhance interaction between the light and the saturable absorbing region 113 inside the GaInP active layer 103. Therefore, it is important to make a sufficient area of the saturable absorbing region 113.
In the above-indicated conventional self-pulsation semiconductor laser, however, the extension of the saturable absorbing region 113 is determined by a small difference between the width W.sub.G ' of the gain region 111 and the width W.sub.P ' of the optical guide region 112 inside the GaInP active layer 103, and the following problem has been involved.
That is, in the conventional self-pulsation semiconductor laser, extension of electric current and extension of light in the transverse direction are controlled by precise control of the thickness d' of the p-type AlGaInP cladding layer 104 underlying the both sides of the stripe portion 107, which results in determining the width W.sub.G ' of the gain region 111 and the width W.sub.P ' of the optical guide region 112. However, since the thickness d' of the p-type AlGaInP cladding layer 104 underlying the both sides of the stripe portion 107 is liable to vary when it is fabricated, the yield of products acceptable for self-pulsated oscillation is low.
Moreover, when the conventional self-pulsation semiconductor laser is operated at a high temperature or for a high output power, self-pulsated oscillation is suppressed due to an excessive transverse extension of the electric current and a reduction of the saturable absorbing region 113.